This invention relates to a method for producing compacts from ceramic or metallic particles and a pressing tool embodying a lower punch, an upper punch and a die for compacting the ceramic or metallic particles.
When compacts are produced from ceramic or metallic powder in a pressing mold the density and density distribution of these compacts depend essentially on the force of pressure and the manner of compression. For compacting one usually employs pressing tools consisting of an upper punch, a lower punch and a die. Since not only the force for overcoming the friction between individual powder grains and their elastic and plastic deformation is necessary for compacting, but also a force for overcoming the friction of the powder on the die wall, an uneven density results in the compact along the height of the pressed powder column. The density is greatest below the pressure ram and smallest in the lower zone of the compact.
Fluctuations in density can be reduced if the pressing tool is formed so that a floating die is moved by the wall friction forces in the pressing direction. This causes the powder column to be compacted by the upper punch and simultaneously pushed against the lower punch and compacted approximately evenly from both sides. Since the die stroke in the pressing direction depends on the frictional conditions and is thus not clearly defined, the resulting density distribution is poorly reproducible. One distinguishes the push-out method from the pull-off method.
In the push-out method the die is mounted on springs and performs a relative motion in the pressing direction during pressing, due to the wall friction forces. When the upper punch returns, the die recoils to its starting position due to the spring force. The compact is thereby raised from the lower punch, so that undesirable splintering or surface flaking can occur on the compact. In the ejection position the compact can be removed.
In the pull-off method predominantly applied today, the die is moved downward in forced coupling with the upper punch during pressing. The relation between the motions of upper punch and die can be coordinated with regard to an even density distribution of the compact. In the discharge position the die is moved down for removal of the compact. The pull-off method largely avoids surface flaking.
A disadvantage of the pull-off method predominantly utilized today is the elaborate kinematics for the upper punch. It is first moved into the closed position and then urged into the pressing position, which means that the upper punch and its drive must be dimensioned accordingly with respect. to both the closing motion and the pressing. Moreover, an elaborate control system is required for the stroke into the closed position and the following pressing motion. The die control is also elaborate, since it must be coordinated with the pressing stroke of the upper punch. The die must thus be moved from the filling position into the pressing position, and held in the pressing position (die support). Moreover, after the upper punch is moved away the die must be moved downward and after the compact is ejected moved up again, which necessitates a complicated structure and complicated control.
Altogether the force-related design of the upper punch kinematics, including the power transmission from the main shaft and the elaborate control of the die, involve a corresponding space requirement as well as a complicated structure of the press itself.
The problem of the invention is to provide a method for producing compacts from powdery material as well as a press for eliminating the above mentioned disadvantages, in particular for realizing a pressing method having simple kinematics and control as well as a simply constructed press.